There are several known pumps of the type having an electric motor and a rotary wheel driven by the motor with a coupling consisting of two groups of permanent magnets to prevent contamination of the fluid being handled. One group of permanent magnets rotates with and is mounted on the shaft of the motor and the other group of magnets is mounted on and rotates with the rotor wheel. In these types of pumps, the interior of the pump is sealed against the environment by means of a diaphragm of nonmagnetic material disposed between the two groups of magnets. The rotor wheel is generally connected to a pump device.
In U.S. Pat. No. 2,970,548 to S. G. Berner, issued Feb. 7, 1961, a magnetically driven centrifugal pump is disclosed. The rotor wheel of the pump is coupled to an electric motor by two concentrically mounted magnets, one on the shaft of the motor and the other on the rotor wheel. Other examples of centrifugal pumps with concentrically mounted magnetic drives are shown in U.S. Pat. No. 3,205,827 to F. N. Zimmerman, issued Sept. 14, 1965 and U.S. Pat. No. 3,238,883 issued to Thomas B. Martin on Mar. 8, 1966. One disadvantage of concentrically mounted magnets is that the diaphragm wall must be made by welding a piece of sheet metal back on itself. However, in welding two thin edges of sheet metal, it is difficult to obtain a satisfactory seam or joint. Furthermore, it is difficult to fabricate the cylindrical wall to such an exact size and shape that the wall everywhere will be flush against the interface near the stator. In view of these considerations, the magnetic gap between concentrically mounted magnets must be substantially greater than comparable axially mounted magnets. Because of the increase in magnetic gap for concentrically mounted magnets, there is an undesirable increase in the loss of magnetic flux through the gap with a corresponding reduction in performance and the additional disadvantage of also requiring larger diameter components to handle higher torque transfers.
In U.S. Pat. No. 2,996,994 to G. W. Wright, issued Aug. 22, 1961, a submersible motor driven pump for pumping liquid fuels utilizing axial gap magnets is disclosed. This motor driven pump utilizes a centrifugal type rotor driven by a sealed motor through a magnetic coupling operating between an imperforate wall of the motor housing. The motor pump is adapted to fit within a variety of fuel tanks. The driving and driven members of the magnetic coupling lie on opposite sides of the imperforate wall, which serves as a rigid diaphragm between the two magnets. Thus, the driven and driving members are separated by an axial air gap. Another example of an axial air gap magnetic motor with a centrifugal pump is disclosed in U.S. Pat. No. 3,223,043 to Harris Shapiro issued Dec. 14, 1965.
Centrifugal pumps have a number of deficiencies. First, they are inherently high speed devices and are more efficient in handling large flows and low pressure rises. Centrifugal pumps have lower efficiencies for small flows and higher pressure rises. Secondly, the pressure rise developed by a centrifugal pump is directly proportional to the speed squared. Thus, centrifugal pumps do not produce high pressure rises at low speed. Third, centrifugal pumps have a tendency to cavitate and lose their prime. When either of these conditions occurs, the centrifugal pump will not pump which may result in generating heat, noise, vibration and the premature failure of the pump.
A further improvement in pumps having axial air gap magnetic drive motors is shown in U.S. Pat. No. 3,470,824 to Elton J. O'Connor, issued Oct. 7, 1969. O'Connor discloses a magnetic drive pump wherein an electrically powered drive motor is sealed from a pump chamber and transmits by electromagnetic forces, a rotary drive to a pump impeller in the pump chamber. The pump has sliding vanes in a fixed casing so that the liquid is directly displaced without requiring the application of centrifugal force.
One major drawback of positive rotary displacement pumps is that their efficiency is dependent on the machining clearances of rotating members. The actual clearance, of course, is a function of the machining and assembly. In addition, with low viscosity liquids, very close tolerances are necessary so as to reduce slippage caused by liquid leaking through the pump clearances. The amount of slip is dependent upon several factors. Generally, increased clearances result in greater slip. Thus, sliding vaned pumps do not find great application in pumping low viscosity liquids since the sliding vanes are prone to excessive tip wear which requires their frequent replacement. In addition, such sliding vane positive rotary displacement pumps are complex, have high friction losses, are expensive to make and do not provide a cut off in case of overpressurization of the fluid handled.
Therefore, none of the aforementioned centrifugal or sliding vane pumps, when used with a magnetic drive coupling between the pump and the electric motor, discloses a pump suitable for handling fuels. In addition, none of the aforementioned pumps are simple, inexpensive to make or provide overpressure protection to limit the discharge pressure of the fluid being handled. Finally, none of the aforementioned pumps are suitable for a multitude of fluids, including fuels, provide high pressure at low speed and voltage, have a low tendency to cavitate, can be easily assembled, and further provide high efficiency.